Airborne sound energy associated with combustion engines, electric fan motors, fans, heating-ventilation-air conditioning (HVAC) systems, intake system, and the like, contributes to noise pollution and is generally undesirable. Noise can be a problem in any place occupied by people, such as within the home, work environment, vehicles, and even personal protective equipment such as respirators. Reducing airborne noise is an especially important in automotive markets. Minimum noise reduction standards for exhaust noise are the subject of numerous government regulations for passenger and commercial vehicles. Further, low cabin noise has long been valuable feature in passenger cars.
Elimination or reduction of sound energy at its source is preferred, but not always possible. In automobiles, for example, airborne sound energy derives from the rapid expansion of internal combustion engine chamber exhaust gases. As these combustion gases are exhausted, a sound wave front travels at sonic velocities through the exhaust system. Automotive noise can also come from cooling fans, alternators and other engine accessories. Accordingly, manufacturers have turned to acoustic technologies capable of substantially reducing the noise emitted by these devices.
The nature of the noise to be reduced is of considerable significance in developing an efficient exhaust or HVAC silencer. The airborne sound energy from a combustion engine or HVAC system typically comes of a plurality of sources, each emitting sound over its own characteristic frequencies. Conventionally, attenuation of a sound wave can be accomplished by causing the wave to encounter surfaces or structures that cause acoustic energy to be dissipated or diverted away from sensitive locations; these interactions turn the individual wave components of high amplitude into a plurality of waves of lesser amplitude, thus lowering the overall noise level. Such devices, to be efficient, may comprise a series of component devices that are individually tuned to alter the phase relationships of respective sound waves.
As described in the literature, perforated films can be used to attenuate sound energy in acoustic silencers. The devices described in these disclosures, however, are generally used for static flow and do not address the effect of such perforated films on the pressure drop associated with the device, as addressed below.